1. Field of the Invention
The present invention relates to tailored-morphology material systems and their use in molecular mass analysis by electromagnetic energy desorption-ionization mass spectrometry. Areas of interest for this technology include, but are not limited to, chemical research and manufacturing, pharmaceutical research and manufacturing, bio-medical research and screening, head-space and environmental monitoring, and other applications involving molecular analysis.
2. Description of the Prior Art
Light desorption-ionization mass spectrometry is a very common and powerful technique for mass analysis of molecules. It is a technique which can be broadened to include the whole spectrum of electromagnetic energy for the desorption-ionization step. However, with recent demands in throughput and small molecule screening, the most popular and widely used laser-based technique, known as MALDI (matrix-assisted laser desorption-ionization), has limitations. MALDI was developed in the mid-eighties and is still being refined today for the analysis of a wide range of compounds with emphasis on proteins, peptides and other molecules in the range of 500–200,000 amu. In MALDI, the analyte (the molecules or compounds to be analyzed) is mixed in with an organic UV absorbent “matrix”. This matrix provides a “soft” method of desorbing large molecules by allowing excess energy in the analyte to be transferred to the matrix molecules during the desorption process. The matrix also provides an environment suitable for the protonation of the analyte molecules, giving them a single, positively charged state. However, for small molecules (approximately 500 amu and below, such as drug molecules), the matrix molecules themselves provide background in the signal and complicate spectrum analysis. Furthermore, with modern demands in automation, throughput and reproducibility, the addition of the matrix to the analyte and its preparation become issues particularly in the case of throughput. These limitations were recognized during the onset of MALDI, leading to the study of non-matrix methods.
The first studies in matrix-less light desorption from a surface used metals and glasses as a media to immobilize the analyte molecules. These materials had non-textured morphologies, i.e., essentially they were non-porous and had a flat (continuous) surface. In a study using this approach, two incident light beams were used, one to desorb and one to ionize the molecules. Zhan, Q. et al., Amer. Soc. Mass Spectrom. 8, 525–531 (1997). This approach is termed two-photon ionization. Other similar methods used ion beams and thermal sources for these tasks. Problems with all these matrix-less light desorption techniques reported in the literature include a high degree of molecular fragmentation and a very limited mass range. These studies, and recent comments, maintain that smooth (non-porous) surfaces do not work effectively for matrix-less laser mass desorption. (See for example, Wei, J., et al., Nature. 399, 243–246 (1999). A recent report supports the understanding that smooth surfaces do not function effectively for matrix-less laser mass desorption. Kruse, R., et al., Anal. Chem. 73, 3639–3645, 2001.
It has been shown that matrix-less laser mass desoroption could be effective if done on a textured surface created with the use of electrochemically etched porous silicon. Wei, J., et al., Nature. 399, 243–246 (1999). With this material as a substrate for laser desorption ionization, significant improvement in non-matrix techniques for molecular analysis has been reported. Also, it was reported that electrochemically etched porous silicon provided mass detection in the range of 0–8000 amu with little fragmentation and little low mass noise. However, other results using this material raised concerns about the low mass collection of hydrocarbons and other contaminants leading to “dirty” low mass signals. Shen, Z., et al., Anal. Chem. 73, 612–619 (2001). The use of HOME-HF electrochemically etched Si, GaAs and GaN, which requires metallic patterning and a wet etching step leading to a porous microstructure, has also been reported for matrix-less laser mass desorption. Kruse, R., et al., Anal. Chem. 73, 3639–3645, 2001.
Further limitations of the electrochemically etched materials are their limited useful lifetimes for mass desorption-ionization applications (<3 weeks) which occur for these materials because of etchants trapped in the material during its manufacturing process. The processing of these etching approaches involves the galvanic etching of a crystal conductive substrate in a hydrofluoric acid based solution. Although the fundamental theory of the mechanism of desorption-ionization of molecules using these techniques is currently under investigation, research groups using these materials reported the importance of the porous structure to the success of mass desoprtion-ionization and reported that solid, smooth (i.e., non-textured) silicon and silicon dioxide coated silicon did not generate ion signals; i.e., were not useful for light desorbed mass spectroscopy.
In other work, liquid matrix materials combined with UV light adsorbing particles have been used in recent laser desorbtion/ionization experiments as an alternative to traditional MALDI matrix materials. Dale et al, Anal Chem, 68, 3321–3329 (1996) used a glycerol/graphite slurry to desorb detect proteins and peptides. This methodology proved less efficient in ionization than traditional MALDI and provided a very noisy spectrum from the glycerol contamination.
The use of a new material, deposited column/void network silicon, for laser desorption-ionization has eliminated several disadvantages associated with electrochemically etched material approaches. Cuiffi, J., et al., Anal. Chem. 73, 1293–1295 (2001). This reported technique of using deposited column/void network materials for mass desorption-ionization produced similar mass ranges and sensitivity to electrochemically etched material, but the film itself did not degrade over time. Furthermore the manufacturability of a deposited film system offers several advantages in cost, production throughput, contamination control, uniformity, and signal reproducibility. This deposited material also offers the further unique feature of having the capability to be deposited on a number of inexpensive substrates, including bio-degradable materials, plastics, and glass. On the other hand, electrochemically etched material always must be on a conducting substrate. In addition, Cuiffi et al. reported, for the first time, that solid (continuous) films of crystalline silicon and thermal silicon dioxide coated crystal silicon did give effective mass desorption-ionization spectroscopy signals. Cuiffi, J., et al., Anal. Chem. 73, 1293–1295 (2001).
The material systems of the present invention consist of one or more deposited film layers and a substrate on which they are deposited. The material system could also be grown (e.g., Si, SiGe alloy, Ge wafer materials) or casted (Si, SiGe, Ge sheet materials) and also function as the substrate. Unlike the previously reported techniques, our deposited material systems offer the flexibility of a number of deposition methods and encompass a broad range of material and morphological choices. These material systems can be uniquely tailored for mass spectrometry applications through choice of the substrate, deposition techniques and materials, deposition parameters and pre- or post deposition physical and chemical modification, which are unavailable in the techniques of electrochemically etched porous silicon whether used with one or two-photon ionization. Specifically, the substrate materials available with our technique are chosen from a group consisting of polymers, plastics, bio-degradable materials, semiconductors, metals, ceramics, insulators, glasses or combinations thereof. Electrochemically or HOME-HF etched porous materials require a conducting semiconductor substrate, and are fundamentally based on a subtractive electric current-driven etching process.
The materials of the present invention can be deposited. This can be done by one or a combination of the additive process comprising physical vapor deposition, chemical vapor deposition, molecular beam epitaxy, plasma assisted physical vapor deposition, plasma enhanced chemical vapor deposition, sol-gel, molecular self-assembly, electroplating, tape casting, spin casting, casting, liquid deposition, or assembly from liquid chemical precursors. The morphology of these materials, which is determined by the production technique and parameters, are application-specific and can range from a continuous (void free) solid with no surface texturing to high surface area to volume ratio (i.e., the deposited column-void nanotextured silicon film), or any intermediate morphologies.
The material systems of the present invention can also be altered by pre or post deposition physical or chemical modifications, which affect the morphology, surface chemistries and bulk material chemistries of the films.
Given these advantages, our material systems are easily integrated, when compared to other matrix-less light desorption/ionization techniques, with microelectronics, micro-fluidics and other micro and nanofabricated sensing devices.
Matrix free desorption/ionization mass spectrometry available using the tailorable morphology of our materials, has a variety of applications. The flexible nature of the substrate material composition, film composition, or both, permitted in our approach allows this technology to be used in atmospheric and reduced pressure desorption and ionization systems as a disposable consumable or reusable target. The composition and methods of production utilized in this technique allow for easy integration with microfabrication processes and microelectronic devices, such as microfluidics, microarrays, CMOS technology and thin film transistors. The matrix less desorption and ionization makes automated, high throughput sample analysis an attractive use of this technique.
The present invention presents a variety of structures that further expand the possibilities of molecular detection using light desorption-ionization, by providing low-cost, easily manufactured, tailorable material systems and techniques.